Canada has a large airspace that offers the US an important line of sight for the US sensors to shoot down missiles from China and Russia. Moreover, historically, Canada has played a big role in air and missile defence for North America.

The president would need to rely on Ottawa to track Chinese and Russian missiles that could come at the U.S. from over the horizon.

The US needs Canadian territory and radar coverage for its Arctic missile defence plan, but political tension and public scepticism may slow cooperation.